The present invention relates to a pickling plant for removing oxide scale on the surface of a fresh hot-rolled steel strip, a method of controlling the same, a continuous through production process for descaling and cold rolling of a plain carbon steel strip, and a continuous through production equipment of descaling and cold rolling of such steel strip.
In general, since the plain carbon steel is rolled at temperatures from 800 to 900.degree. C., oxide scale of black color comprising Fe.sub.3 O.sub.4 as a major component may be produced. The scale is pressed into the surface layer of the steel strip during a following cold rolling process to cause a scale scratch on the strip. Thus, scale should be indispensably removed. The catenary type pickling process is most widely used as a method of removing scale from the plain carbon steel strip, in which a steel strip is dipped in a tank containing a solution of hydrochloric acid and continuously passed through the tank to remove the scale by chemical reaction. The hydrochloric acid solution is supplied into the tank from the downstream side of the running strip and discharged from the upstream side thereof, wherein the acid concentration of the hydrochloric acid pickling solution is decreased to approximately 3 to 5%, while the concentration of FeCl.sub.2 is increased an order of ten and several percent. The solution is recycled to a hydrochloric acid solution having a concentration of about 18% through the ARP (acid-recovering system) and is led to the downstream side of the pickling tank. In general, the capacity of the ARP is set on the basis of the Fe ion concentration of 120 g/liter in the pickling solution recovered from the downstream. The value of 120 g/liter corresponds to the concentration of Fe ion contained in the pickling solution when all the scale is entirely dissolved just at the downstream side end of the pickling tank with respect to the running strip.
In the case where the line speed is required to be slowed down, studying an unusual operation, for example, if the temperature of the pickling solution remains unchanged as it is, descaling is completed prior to the down stream side end of the picking tank with respect to the running strip resulting in that the steel of the exposed base surface of the steel strip is immersed in the acid solution for a too long time, which is called as "over picking" because the immersion time of the steel in the pickling solution is prolonged. In such the case, not only the base surface of the steel is roughened to deteriorate the quality of the product, but also the pickling solution is consumed too much and deteriorated unsuitably. On the other hand, in the case where steel strips different in thickness of scale to one another is introduced into the production line, there is a possibility that a steel strip having a thick oxide film goes out of the pickling tank without completion of descaling, while a steel strip having a thin oxide film is subjected to over pickling like the case where the line speed is slowed down, provided that the operating conditions are the same to one another.
In the known practice, it has not yet been established how to handle the above respective cases. For example, in the case where the line speed is decreased, operating conditions have been determined in accordance with experiences such as effectively lowering the operation temperature. But, it has not been clear how low the operation temperature in pickling should be, and an optimum operating condition has been determined in accordance with observation of the surface state of the steel strip which comes out from the pickling tank.
JP-A-59-209415 discloses a descaling method in which the pickling speed is controlled on the basis of coiling temperature of a hot rolled steel strip. JP-A-62-196385 discloses a descaling method in which a hot rolled steel strip is passed through molten salt and subsequently subjected to pickling, wherein the flow rate of the acid solution supplied in the pickling tank is controlled on the basis of the temperature of the steel strip before entering into the molten salt. JP-A-1-254313 discloses a descaling method in which, when a hot rolled steel strip is subjected to pickling, the supply rate of the acid solution from a nozzle into the pickling tank is controlled on the basis of the thickness and a material type of the strip and hot rolled conditions of the strip. JP-A-6-212462 discloses a pickling method in which the concentration and the temperature of the acid solution are changed on the basis of the state of the oxide film on the surface of the steel strip. However, none of the above publications teaches a distribution of acid concentration in the pickling tank, a distribution of Fe ion concentration in the pickling tank and an efficiency of descaling.